4.1 Theoretical considerations on impacts of oxy-fuel fir- fir-ing on SO x and its capture
4.1.1 SO 2 concentration levels in air and oxy-fuel combustion
4.1 Theoretical considerations on impacts of oxy-fuel
removal47; this corresponds to recycle option a in figure 2.2). It is important to differentiate between wet and dry concentrations when discussing the differences between air and oxy-fuel operation. The reason for this is that commonly flue gas concentrations are measured and reported on a dry basis. Consequently, when comparing SO2 concentration levels of air and oxy-fuel processes, concentrations on dry basis are commonly used (e.g. [1, 9, 10, 13]). However, for the thermodynamic stability of sulfates in furnaces the actual conditions in this unit are of relevance and hence, the SO2concentrations on a wet basis need to be considered. To discuss differences of SO2 concentration levels between air and oxy-fuel firing, the ratio between ySO2,max,oxy andySO2,max,air can be calculated (i.e. ξoxy/air,dry =ySO2,max,oxy,dry/ySO2,max,air,dry and ξoxy/air,wet =ySO2,max,oxy,wet/ySO2,max,air,wet), giving the expected increase of wet and dry concen-trations in oxy-fuel operation, compared to air firing.
ySO2,max,air,wet =
γS MM,S γW
MM,W + γH
2MM,H + γS
MM,S + γC
MM,C + γN
2MM,N +0.791 +yO2,exc,dry 0.209−yO2,exc,dry
· · ·cont.· · ·
· · ·cont.· · ·
·* . ,
γS
MM,S + γC
MM,C + γH
4MM,H − γO 2MM,O+
/
- (4.3)
ySO2,max,oxy,wet =
γS MM,S γW
MM,W + γH
2MM,H + 1
1−yO2,exc,dry−yleak,dry
· · ·cont.· · ·
· · ·cont.· · ·
· * . ,
γS
MM,S + γC
MM,C + γN 2MM,N +
V˙CO2 M˙RCVmol
+ /
-(4.4)
As can be seen from equations 4.1, 4.2, 4.3, and 4.4, for one fuel quality theseξoxy/airratios for wet and dry concentrations are influenced by the oxygen excess (yO2,exc,dry), the air ingress to the oxy-fuel process (yleak,dry), and the specific volumetric flow of clean CO2 (i.e.vCO∗
2,dos =V˙CO2/M˙RC) used for fuel dosing48. When comparingξoxy/air values for similar process conditions (i.e.
sameyO2,exc,dry,yleak,dry, andvCO∗
2,dos ranges) but different fuel qualities, one sees that also the
47The cleaned primary recycle gas is considered in the calculations by assuming a certain fuel specific amount of clean CO2vCO∗
2,dos for fuel feeding.
48In the experiments at IFK’s KSVA this is clean CO2from a tank. In industrial facilities (e.g. “Schwarze Pumpe”
oxy-fuel pilot plant) it is cleaned flue gas that is recirculated.
0.0 2.5 5.0 7.5 10.0 2.5
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
yO2,exc,dryin10−2 mm33
ξair/oxy
L1, dry L2, dry L3, dry C1, dry L1, wet L2, wet L3, wet C1, wet
(a)Excess O2: Lignites, coal C1
0.0 2.5 5.0 7.5 10.0
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
yO2,exc,dry in10−2 mm33
ξair/oxy
C2, dry C3, dry C4, dry C2, wet C3, wet C4, wet
(b)Excess O2: Coals C2, C3, C4
0 5 10 15
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
yleak,dryin10−2 mm33 ξair/oxy
L1, dry L2, dry L3, dry C1, dry L1, wet L2, wet L3, wet C1, wet
(c)Air ingress: Lignites, coal C1
0 5 10 15
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
yleak,dry in10−2 mm33 ξair/oxy
C2, dry C3, dry C4, dry C2, wet C3, wet C4, wet
(d)Air ingress: Coals C2, C3, C4
0.0 0.3 0.5 0.8 1.0
2.53.0 3.54.0 4.55.0 5.56.0 6.5
vCO∗
2,dos in mkg3,ST P ξair/oxy
L1, dry L2, dry L3, dry C1, dry L1, wet L2, wet L3, wet C1, wet
(e)Clean primary CO2: Lignites, coal C1
0.0 0.3 0.5 0.8 1.0
2.53.0 3.54.0 4.55.0 5.56.0 6.5
vCO∗
2,dos in mkg3,ST P ξair/oxy
C2, dry C3, dry C4, dry C2, wet C3, wet C4, wet
(f)Clean primary CO2: Coals C2, C3, C4 Figure 4.1: ξoxy/air ratios (wet and dry) betweenySO2,max,oxy andySO2,max,air calculated for the tested fuels and relevant ranges of the process parametersyO2,exc,dry,yleak,dry, and vCO∗
2,dos (a, b: yleak,dry=5·10−2 mm33,vCO∗
2,dos=0.6mkg3; c, d: yO2,exc,dry=4·10−2 mm33,vCO∗
2,dos= 0.6mkg3; e, f: yO2,exc,dry=4·10−2 mm33,yleak,dry=5·10−2 mm33).
fuel composition altersξoxy/air values. In this respect,ξoxy/air,wet is strongly impacted by the moisture content of the fuel. The contents of carbon, oxygen, hydrogen, sulfur, and nitrogen in the fuel influenceξoxy/air,wet as well asξoxy/air,dry, even though, the impact of sulfur and nitrogen is generally low, due to their relatively low contents in most fuels.
To illustrate the ranges ofξoxy/air expected in practice for the fuels tested in the framework of this thesis and for practically relevant ranges ofyO2,exc,dry,yleak,dry, andvCO∗
2,dos,ξoxy/air ratios are plotted in figure 4.1. It can be seen that the obtainedξoxy/air ratios for all the lignites are similar, as their elemental compositions are. Due to its similar behavior, coal C1 is plotted in diagrams together with the lignites (figures 4.1a, 4.1c, and 4.1e). Theξoxy/air ratios for the coals C2, C3, and C4 are similar and both,ξoxy/air,dry andξoxy/air,wet ranges are higher than those of the lignites and coal C1, with coal C4 showing the highest difference between the SO2 concentrations of air and oxy-fuel firing. Another observation is that ξoxy/air,wet and therefore, the thermodynamically relevant increase between the air and oxy-fuel SO2levels is always lower thanξoxy/air,dry. This is due to the higher H2O levels in wet recycle oxy-fuel firing, compared to air combustion that dilute concentrations of other components, such as SO2. When comparing the plots of figure 4.1, it is obvious thatξoxy/air is more sensitive towards changes ofyO2,exc,dry and ofvCO∗
2,dos than towards changes ofyleak,dry. ξoxy/air increases with yO2,exc,dry, while it drops with increasingvCO∗
2,dos. The reason for this is that an increase of the excess oxygen in air firing goes along with additional dilution by airborne nitrogen, which is not the case in oxy-fuel conditions. In contrast, increasing the specific amount of clean CO2only dilutes the SO2concentrations in oxy-fuel operation, without any impact on the air fired concentration levels. For the tested fuels and an air and comparable practical oxy-fuel setup (i.e. non-ideal system with air ingressyleak,dry of 5·10−2 mm33, cleaned primary recycle gas for milling/dosing of fuelvCO∗
2,dos of 0.6mkg3, and an oxygen flue gas concentrationyO2,exc,dry of 4·10−2 mm33), the SO2concentrations can be expected to increase by a factor of 3.4-4.2 referring to dry and of 2.9-3.5 for wet conditions. For an idealized oxy-fuel setup with no air ingress and no utilization of cleaned primary recycle gas for milling/dosing of fuel (yO2,exc,dry= 4·10−2 mm33) the ranges forξoxy/air,dry andξoxy/air,wet are 5.9-6.5 and 4.2-4.9, respectively. Theξoxy/air,dry
ratios for a practical oxy-fuel setup compare well with those previously reported by others from experiments of 2-4 [1, 9, 13].